L-amino acid is the basic unit of protein and is widely used as a functional food additive and a nutrient source for animals and in the pharmaceutical industry. Among 20 amino acids, branched-chain amino acids consist of three members, L-valine, L-leucine and L-isoleucine, and the industrial value thereof is gradually increasing. It was reported that branched-chain amino acids play an important role in maintaining and forming human skeletal muscle, and functioning to regulate insulin, and maintaining and increasing muscle mass (Andrea tom et al, (2006) The journal of nutrition, 136, 324s-330s). Particularly, L-isoleucine is metabolized in muscle to produce energy and is involved in hemoglobin production, and reduces fatigue and promotes growth. Thus, it is used in various applications, including injectable fluids and nutrients, and its use in sport nutritional foods is also increasing.
To industrially produce L-isoleucine, Corynebacterium glutamicum is used as a representative microorganism. This microorganism produces L-isoleucine via three intermediate metabolites from pyruvate and 2-ketobutyrate as precursors (see FIG. 1). From the two precursors, 2-aceto-2-hydroxybutyrate is synthesized, and 2,3-dihydroxy-3-methylvalerate and 2-keto-3-methylvalerate are synthesized therefrom, and L-isoleucine is finally produced. To produce each of the metabolites, the enzymes acetohydroxy acid synthase, acetohydroxy acid isomeroreductase, dihydroxy acid dehydratase and aminotransferase are used (Jin Hwan Park et al, Appl microbial biotechnol, (2010) 85:491-506).
In Corynebacterium glutamicum strains, acetohydroxy acid synthase that is important in the L-isoleucine biosynthesis step is encoded by the ilvBN gene, and undergoes feedback inhibition by the final product L-isoleucine so that the expression of the gene and the activity of the enzyme are inhibited. In addition, threonine dehydratase that produces 2-ketobutyrate also undergoes feedback inhibition by L-isoleucine. Thus, it is known that the regulation of expression of genes and activity of enzymes involved in L-isoleucine biosynthesis are critical to generating strains that produce L-isoleucine in high yield (Jin hwan park et al, Biotechnology journal, (2010) 560-577). In addition, as can be seen in FIG. 1, L-isoleucine, L-valine and L-leucine are produced through the same biosynthesis pathway. Thus, in order to mass-produce L-isoleucine, L-threonine, that is used as a precursor of 2-ketobutyrate, should be sufficiently supplied so that the production of other branched-chain amino acids can be reduced and L-isoleucine can be continuously produced. In an attempt to solve this issue, it was reported that α-amino-β-hydroxynorvaline, an L-threonine derivative, could be used to increase the production of L-threonine (Cayo Ramos et al, Applied and environmental microbiology, (1992) 1677-1682). Further, a method of imparting L-isoleucine production ability to a microorganism having the ability to produce L-threonine (Korean Patent Laid-Open Publication No. 2011-0058731), a microorganism that produces L-threonine and L-isoleucine at the same time (Korean Patent Laid-Open Publication No. 2002-0013777), etc., were reported. Also, it was reported that the use of 4-thiaisoleucine, an isoleucine derivative, inhibited the feedback of threonine dehydratase (John J. Wasmuth, Journal of bacteriology, (1973) 562-570). Moreover, it was reported that a mutant strain resistant to isoleucine-hydroxamate has an enhanced ability to produce L-isoleucine (M. Kisumi, Journal of general microbiology, (1971) 69 291-297). In addition, there were reports of an R&D method for AHAS that comprises mutating an L-isoleucine-producing strain to increase the production of L-isoleucine compared to the production of L-valine (Korean Patent Laid-Open Publication No. 2011-0061780), and a study focused on increasing the production yield of L-isoleucine by changing the supply of oxygen, or physical conditions such as pH during fermentation (Zhihian Peng et al, Bioprocess biosyst eng, (2010) 33:339-345).
However, L-isoleucine-producing microorganisms, which have been studied and developed to date, are separately resistant to some substances in the L-isoleucine biosynthesis pathway. Thus, there still remains a need to develop an L-isoleucine-producing microorganism resistant to various substances that are involved in the control of feedback in L-isoleucine biosynthesis.